This invention relates to a process for displacing oil within a subterranean reservoir. More particularly, it relates to improved surfactant materials, compositions, and operating procedures that are useful in waterflood oil recovery processes, especially in reservoirs having relatively high temperatures. The surfactant materials provided by this invention are generally useful whenever a surfactant material is useful.
In a waterflood oil recovery process, an aqueous liquid which is preceded by liquid containing an active surfactant system is injected into the reservoir to displace oil toward a location from which the oil is recovered. In order to increase the efficiency of the oil displacement, the frontal portion of the injected aqueous liquid is arranged to contain or to be preceded by an "active" surfactant system. An active surfactant system is one which has an interfacial tension against oil that is at least as low as 10.sup.-.sup.2 dyne per centimeter and is preferably as low as 10.sup.-.sup.4 dyne per centimeter. In such a system the surface active components may be dissolved or dispersed in aqueous liquid, oil, or a two-phase liquid system in which the continuous (or external) phase is either water or oil. In general, the oil displacing efficiency of an active surfactant system is such that the injection of one pore volume is sufficient to displace at least about 90 percent of the residual oil from a permeable earth formation material in which the oil displacement efficiency is not limited by the extent of adsorption of the surfactant.
In a waterflood oil recovery process, the efficiency of the oil displacement is strongly affected by the activity (extent of lowering of the oil/water interfacial tension) of the surfactant system. In such a process, the concentration of surface active material in the surfactant system is generally kept low, usually not much more than about 10 percent by weight, in order to minimize the cost of the system. Because of this, it is important that the surfactant system be one from which there is relatively little loss of surfactant during an oil recovery operation. In an oil recovery operation, the surfactants tend to be lost due to (for example): the precipitation of surfactants as insoluble salts of materials, such as polyvalent metal ions, that may be dissolved in the fluid in the reservoir; the adsorption of surfactant material on the reservoir rocks; and/or chemical conversions of the surfactant materials, such as hydrolysis of a sulfate that is an active component of the surfactant system to an alcohol that is insoluble or inactive in that system, etc.
In relatively shallow reservoirs (e.g., reservoirs at depths within a few thousand feet below the surface of the earth), the reservoir temperatures are generaly less than about 150.degree. F. In such reservoirs, the surfactant systems of the above-identified copending patent application are particularly advantageous with respect to having a relatively low cost and providing a high degree of activity and relatively low rate of surfactant loss due to precipitation, adsorption and/or hydrolysis. A typical surfactant system of that type comprises an aqueous saline solution containing a mixture of at least one petroleum sulfonate surfactant and at least one alkoxylated alcohol sulfate surfactant. When the sulfate components of such a system are hydrolyzed, they tend to be converted to alcohols that are much less soluble and active as surfactants. In an active surfactant system, the hydrolysis of such a sulfate surfactant usually causes it to become ineffective in respect to being a contributor to the oil displacing efficiency of the system.
The tendency for a sulfate surfactant to undergo hydrolytic decomposition is strongly affected by an increase in temperature. For example, the sodium sulfates of ethoxylated, linear, primary aliphatic alcohols containing from about 12 to 15 carbon atoms and about 3 ethylene oxide groups per molecule are very effective surfactants in aqueous saline surfactant systems. Athough these sulfates exhibit a half-life in the order or about 4000 days at temperatures near 150.degree. F, their half-life diminishes to a mere 200 days at a temperature near 190.degree. F. The duration of a waterflood oil recovery operation often extends throughout several years.